In recent years, surface mount technology with high density has been widely adopted. Such surface mount technology are classified, among others, into double-sided surface mount technology in which chip type parts are joined with use of solder paste, and hybrid mount technology which is a combination of surface mount technology of chip type parts using solder paste and through-hole mount technology of discrete parts. In either mount process, a printed wiring board is subjected to two or more soldering steps, and thus it is exposed to high temperatures resulting in a severe thermal history.
Oxide film formation is accelerated by heating the surface of copper or copper alloys constituting the circuit part of a printed wiring board, and thus the surface of the circuit part cannot maintain good solderability.
In order to protect the copper circuit part of the printed wiring board from air oxidation, a chemical layer is generally formed on the surface of the circuit part using a surface treating agent. It is necessary, however, that good solderability be maintained by preventing the chemical layer from degenerating (i.e., being degraded) to protect the copper circuit part even after the copper circuit part has a thermal history of multiple cycles.
Tin-lead alloy eutectic solders have been conventionally used for mounting electronic parts to a printed wiring board, etc. In recent years, however, concerns have developed that the lead contained in the solder alloy adversely affects the human body, and thus the use of lead-free solder is desired.
Accordingly, various lead-free solders are being considered. For example, lead-free solders have been suggested in which one or more metals, such as silver, zinc, bismuth, indium, antimony, copper, etc., are added to a base metal of tin.
The conventionally used tin-lead eutectic solder is excellent in wettability on the surface of substrate, particularly copper, and thus strongly adheres to copper, resulting in high reliability. In contrast, lead-free solder is inferior to the conventionally used tin-lead solder in wettability on a copper surface, and thus exhibits poor solderability and low bonding strength due to voids and other bonding defects.
Therefore, when using lead-free solder, it is necessary to select a solder alloy with superior solderability and a flux which is suitable for use with lead-free solder. A surface treatment agent for use in preventing oxidation on the surface of copper or a copper alloy is also required to have functions for improving the wettability and solderability of the lead-free solder.
Many lead-free solders have a high melting point, and a soldering temperature that is about 20 to about 50° C. higher than that of the conventionally used tin-lead eutectic solder. Thus, surface treatment agent for use in the process of soldering with lead-free solder should have the characteristic of being able to form a chemical layer with excellent heat resistance.
As active ingredients of these surface treating agents, various imidazole compounds have been proposed. For example, Patent Document 1 disclose 2-alkylimidazole compounds such as 2-undecylimidazole; Patent Document 2 discloses 2-arylimidazole compounds such as 2-phenylimidazole and 2-phenyl-4-methylimidazole; Patent Document 3 discloses 2-alkyl-benzimidazole compounds such as 2-nonylbenzimidazole; and Patent Document 4 discloses 2-aralkylbenzimidazole compounds such as 2-(4-chlorophenylmethyl)benzimidazole, respectively.
However, in the case where a surface treating agent containing such an imidazole compound is used, the heat resistance of a chemical layer as formed on the copper surface was not satisfactory yet. Also, in soldering, the solder wettability is not sufficient so that good solderability cannot be obtained. In particular, in the case of performing soldering using a lead-free solder in place of the eutectic solder, it was difficult to put the foregoing surface treating agent into practical use.    [Patent Document 1] JP-B-46-17046    [Patent Document 2] JP-A-4-206681    [Patent Document 3] JP-A-5-25407    [Patent Document 4] JP-A-5-186888